The present invention relates to a butyl rubber composition, and more particularly to a butyl rubber composition for effective use as molding materials of sealing materials for air conditioners using carbon dioxide as a refrigerant.
Heretofore, flon-series gases have been used as air conditioner refrigerants, and it is known that the flon-series gases can destroy global environments due to their chemical structures, for example, by destroying the ozonosphere or by warming the earth""s atmosphere. Thus, studies have been extensively made on earth-friendly refrigerants, one of which is carbon dioxide (CO2). CO2 is highly soluble in polymer materials, resulting in permeation into sealing materials or blister generation due to changes in pressure in the use circumstances, etc. It is in the present status that no sealing materials capable of confining CO2 therein have been found yet.
An object of the present invention is to provide a butyl rubber composition capable of giving vulcanization-molded products capable of effectively confining a carbon dioxide refrigerant therein without blister generation.
The object of the present invention can be attained by a butyl rubber composition, which comprises 100 parts by weight of butyl rubber and 30 to 150 parts by weight of carbon black having a CTAB specific surface area (according to ASTM D 3765) of 30 to 100 m2/g. It is preferable that the butyl rubber composition contains 10 to 100 parts by weight of a flat filler having as an average particle size of 1 to 40 xcexcm and an aspect ratio of 5 or more and 0.05 to 5 parts by weight of a coupling agent of organometallic compound.
Butyl rubber is a copolymer of isobutylene and isoprene, where at most 3% by mole of isoprene is copolymerized. In the present invention, it is preferable to use a copolymer containing about 2% by mole of isoprene.
Carbon black having a CTAB (cetyltrimethyl ammonium bromide) specific surface area of 30 to 100 m2/g, preferably 35 to 95 m2/g, is contained in butyl rubber in an amount of 30 to 150 parts by weight, preferably 50 to 130 parts by weight, on the basis of 100 parts by weight of butyl rubber. When carbon black having a CTAB specific surface area of more than 100 m2/g is used, the reinforcing effect is too strong, resulting in increased rubber compound viscosity or dispersion failure, whereas carbon black having a CTAB specific surface area of less than 30 m2/g is used, no practical level reinforcing effect is obtainable and bubbles are formed upon contact with the CO2 refrigerant. This is also true in the case of a lower blending proportion than 30 parts by weight as in the case of a lower CTAB specific surface area of 30 m2/g, whereas in the case of a higher blending proportion than 150 parts by weight, not only kneading will be difficult to conduct due to the resulting increased rubber compound viscosity but also very hard vulcanization molded products will be produced.
When a flat filler is used together with carbon black having such specific surface areas, shieldability against the carbon dioxide refrigerant can be improved. The flat filler includes, for example, clay, mica (mica powder), graphite, molybdenum disulfide, etc., and is used in a proportion of 10 to 100 parts by weight, preferably 30 to 80 parts by weight, to 100 parts by weight of butyl rubber.
The flat filler having an average particle size of 1 to 40 xcexcm, preferably 5 to 30 xcexcm, and an aspect ratio of 5 or more, preferably 10 to 30, can be used. When the average particle size is less than 1 xcexcm or the aspect ratio is less than 5, no improvement of the refrigerant shieldability is observable, whereas when the average particle size is more than 40 xcexcm, not only the practical level reinforcing effect cannot be obtained, but also blister generation appears upon contact with the carbon dioxide refrigerant. When the blending proportion is less than 10 parts by weight, not improvement of the refrigerant shieldability is observable. When the blending proportion is not less than 100 parts by weight, blister generation appears upon contact with the carbon dioxide refrigerant, as in the case where the larger average particle size is used.
In the case of using a flat filler for an improvement of the refrigerant shieldability, it is necessary to use 0.05 to 5 parts by weight, preferably 0.1 to 3 parts by weight, of a coupling agent of organometallic compound at the same time. When the blending proportion is less than 0.05 parts by weight, no coupling effect is observable, and blister generation appears upon contact with the carbon dioxide refrigerant. On the other hand, when the blending proportion is more than 5 parts by weight, cross-linking inhibition appears and the compression set characteristic is deteriorated.
Coupling agent of organometallic compound includes, for example, silane coupling agents such as vinyltrimethoxysilane, vinyltriethoxysilane, vinyl tris(2-methoxyethoxy)silane, vinyl-trichlorosilane, xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropyltriethoxysilane, xcex3-glycidoxypropylmethyl-diethoxysilane, xcex3-methacryloxypropyltrimethoxysilane, xcex3-methacryoloxypropyltri-ethoxysilane, xcex3-methacryloxypropylmethyldimethoxysilane, xcex3-methacryoxypropylmethyldiethoxysilane, etc.; titan coupling agents, such as isopropyltriisostearoyl titanate, isopropyltris(dioctylpyrophosphate) titanate, isopropyltri(N-aminoethyl-aminoethyl) titanate, tetraoctyl bis(ditridecylphosphite) titanate, bis(dioctylpyrophosphate) oxyacetate titanate, bis(dioctylpyrophosphate) ethylene titanate, etc.; and aluminum coupling agents such as aluminum ethylate, aluminum isopropylate, aluminum isopropylate mono-sec-butyrate, aluminum sec-butyrate, aluminum alkylacetoacetate diisopropylate, aluminumtrisacetyl acetonate, aluminum alkylacetoacetate, etc., which can be used alone or in cominiation of two or more.
Any of such cross-linking agents as sulfur (donor), morpholine, quinoid, halogenated alkylphenol formaldedhyde resin, etc. can be used, so long as they are applicable to butyl rubber. Butyl rubber composition can contain any other appropriate additives than those mentioned above, for example, a reinforcing agent such as white carbon, etc., a filler such as talc, clay, graphite, calcium silicate, etc., a processing aid such as stearic acid, palmitic acid, paraffin wax, etc., an acid receptor such as zinc oxide, magnesium oxide, etc., an antioxidant, a plasticizer, etc., if desired.
The composition can be prepared by kneading through a mixer such as Intermixer, a kneader, Bambury mixer, etc., or open rolls. Vulcanization thereof can be carried out by heating at about 150xc2x0 C. to about 200xc2x0 C. for about 3 to about 60 minutes through an injection molding machine, a compression molding machine, a vulcanization press, etc. Oven vulcanization (secondary vulcanization) can be carried out, if desired, by heating at about 120xc2x0 to about 200xc2x0 C. for about 1 to about 24 hours.
The present invention will be described below, referring to Examples.